The present invention relates to a model-based method of estimating the crankcase oil pressure of an internal combustion engine.
An accurate indication of crankcase oil pressure in an internal combustion engine is useful for control purposes, such as for scheduling the gain of control algorithms for systems that are sensitive to oil pressure fluctuations. For example, in an engine equipped with a hydraulically activated variable valvetrain system, the control algorithm gains should be matched with the actuator response time, which varies with engine oil pressure. Although the engine oil pressure may be measured directly with a dedicated sensor, the usual oil pressure sensors are not very accurate, and most automotive manufacturers rely on an estimate of the oil pressure to avoid the expense of a better sensor when a more accurate pressure indication is needed. For example, the oil pressure that occurs for different steady-state operating conditions of the engine (different values of engine speed, or different combinations of engine speed and oil temperature, for example), can be measured and stored in a look-up table for subsequent retrieval during operation of the engine. However, such estimation techniques typically require extensive calibration effort, and tend to perform poorly during transient engine operating conditions. Accordingly, what is needed is an estimation method for use in a production vehicle that is relatively easy to calibrate and that provides an accurate estimation of the engine oil pressure during both steady-state and transient engine operating conditions.
The present invention is directed to an improved method of estimating the crankcase oil pressure of an internal combustion engine with a physical model that takes into account both engine speed and oil temperature. According to the invention, the oil pressure is estimated as the sum of static and dynamic components, where the dynamic component includes a first portion that primarily models flow-related effects, and a second portion that primarily models temperature-related effects. In a preferred embodiment, the constants and temperature-related variables of the physical model are combined to form one constant calibration value, and two temperature-dependent calibration values. The parameters of the physical model provide a starting point for the calibration values, and the calibration values are then tuned during a calibration procedure so that the estimated oil pressure tracks an accurate measure of the oil pressure during both steady-state and transient engine operating conditions.